Related fields include semiconductor devices and their fabrication; in particular, thin-film components of resistive-switching non-volatile memory (ReRAM).
Nonvolatile memory elements are used in computers and other devices requiring persistent data storage (e.g., cameras, music players). Some traditional nonvolatile memory technologies (e.g., EEPROM, NAND flash) have proven difficult to scale down to smaller or higher-density configurations. Therefore, a need has developed for alternative nonvolatile memory technologies that can be scaled down successfully in terms of performance, reliability, and cost.
In resistive-switching-based nonvolatile memory, each individual cell includes a bistable variable resistor. It can be put into either of two states (low-resistance or high-resistance), and will stay in that state until receiving the type of input that changes it to the other state (a “write signal”). The resistive state of the variable resistor corresponds to a bit value (e.g., the low-resistance state may represent logic “1” and the high-resistance state may represent logic “0”). The cell is thus written to by applying a write signal that causes the variable resistor to change resistance. The cell is read by measuring its resistance in a way that does not change it.
ReRAM cells include layers of different materials: for example, cells using oxygen-vacancy-based switching may include dielectric oxides, conductive metals or metal nitrides, and resistive ternary compounds such as metal silicon nitrides. Annealing, forming the first oxygen-vacancy filament, and subsequent long-term operation may cause atoms from one layer to diffuse into another and undesirably change its electrical properties.
For example, if metal atoms diffuse into a metal-oxide switching layer, they may distort the oxygen-vacancy filaments or form competing filaments of their own that might not easily break in response to the write signal. Metal atoms may also add conductivity to the layer so that the low-resistance state and the high-resistance state are both shifted downward and may become difficult to read.
Conversely, when oxygen diffuses into electrodes, interconnect lines and other conductive layers, it can compromise their conductivity. As a result of the added resistance, the cell may require more operating power and dissipate more waste heat. In addition, source electrodes in an oxygen-vacancy ReRAM cell are intended to donate and accept oxygen vacancies to and from the switching layer(s). If the source electrode collects too much diffused oxygen, it may become less effective at trading vacancies with other layers, and the difference between the low-resistance state and the high-resistance state may diminish, becoming more difficult to distinguish and possibly causing reading errors.
Therefore, a need exists for preventing diffusion of oxygen and metal through ReRAM cells. To accommodate continued miniaturization, the prevention of diffusion should preferably not add significant excess thickness to the cells.